Exoplanet Wars: “First Habitable World” May Not Exist

The long-sought planet, dubbed Gliese 581g, was detected using a combination of 122 observations over 11 years from the Keck Telescope in Hawaii, and 119 published measurements from the HARPS spectrograph that spanned 4.3 years, up until 2008. The HARPS team had already found four other planets circling Gliese 581 by teasing out the star’s subtle motion in response to the planets’ gravitational tugs.

On Sept. 29, Steven Vogt of the University of California, Santa Cruz and R. Paul Butler of the Carnegie Institution of Washington announced that adding their decade of observations to the data pool pointed to the existence of two more planets. One of the new planets sits squarely in the star’s habitable zone, where liquid water could persist at the planet’s surface and life could find a foothold.

But not so fast, says the HARPS team. Astronomer Francesco Pepe of the Geneva Observatory in Switzerland, who spoke Oct. 11 at an International Astronomical Union symposium on planetary systems, reported a new analysis using only HARPS data, but adding an extra 60 data points to the observations published in 2008. He and his colleagues could find no trace of the planet.

“I am not overly surprised by this as these are very weak signals, and adding 60 points onto 119 does not necessarily translate to big gains in sensitivity,” Vogt told Wired.com in an e-mail. He added that the non-detection doesn’t mean the planet doesn’t exist — it may be that it’s only detectable using both data sets.

“I do not wish to comment on Pepe’s result as I haven’t seen his data,” Vogt said. “But I have tremendous respect for their work, and look forward to seeing their additional data on this and other systems of mutual interest.”